Fuel cells have been developed as alternative power sources for motor vehicles, such as electrical vehicles. A fuel cell is a demand-type power system in which the fuel cell operates in response to the load imposed across the fuel cell. Typically, a liquid hydrogen containing fuel, for example, gasoline, methanol, diesel, naphtha, etc. serves as a fuel supply for the fuel cell after the fuel has been converted into a gaseous stream containing hydrogen. The conversion to the gaseous stream is usually accomplished by passing the fuel through a fuel reformer to convert the liquid fuel to a hydrogen gas stream that usually contains other gases such as carbon monoxide, carbon dioxide, methane, water vapor, oxygen, and unburned fuel. The hydrogen is then used by the fuel cell as a fuel in the generation of electricity for the vehicle.
A polymer electrolyte membrane type of fuel cell is generally composed of a stack 10 of unit cells 72 comprising a polymer electrolyte membrane 11 enclosed between electrodes 12 and gas diffusion layers 13, and further enclosed between separators 15 and channels 14 for fuel gas and oxidant gas, as shown in FIG. 1. The stack 10 is fixed by end plates 16. A current collector may be provided between the end plate and stack, or the end plate 16 itself may function as current collector. When hydrogen is used as the fuel gas and oxygen is used as the oxidant gas, electrons are released due to a chemical reaction occurring at catalyst reaction sites on the electrode surfaces. Water is formed as a by-product, via the reaction:H2+½O2→H2O.
Consequently, the fuel cell is an energy source that has no adverse impact on the global environment, and has been the focus of much research for use in automobiles in recent years.
From the standpoint of durability, fuel cell electrical generating performance deteriorates over its operating life, due to a build-up of impurities such as metallic ions and organics in the fuel cell. The impurities result from various sources: for example, they may be extracted from tubing used to supply gas or coolant to the fuel cell, or from auxiliary equipment. In addition, there may be impurities mixed with the fuel gas or oxidant gas. It is possible to reduce the concentration of impurities by using material that does not contain impurities for tubing or auxiliary equipment, or by filtering the fuel gas and oxidant gas. However, when generating electricity over a long period of time, it is difficult to prevent the accumulation of impurities inside the fuel cell and the accompanying deterioration of fuel cell performance. Impurities inside the fuel cell adhere to catalytic reaction sites and causes loss of catalytic performance.
There are known methods of re-activating the catalyst by electrochemically removing the impurities that adhere to it. U.S. Pat. No. 6,187,464, for example, describes a method of generating electricity in a polymer electrolyte fuel cell module at an oxygen utilization rate of 50% or higher, and impressing on the fuel cell module an average voltage of 0.3 V or less per unit cell. Japanese Patent Disclosure 2001-85037 describes another method of restoring fuel cell performance by operating the fuel cell at a current density 1.5 times greater than the normal operating current density or by reversing the direction of current flow.